PROJECT SUMMARY/ABSTRACT Over 50 million individuals suffer from arthritis in the US. Joint pain due to arthritis causes major disabilities, leading to 172 million missed workdays and $156 billion in lost wages and medical costs each year. Although current treatments can alleviate some clinical symptoms, these approaches cannot cure the underlying irreversible cartilage loss. Stem cell therapy can restore pain-free joints by regenerating rather than repairing cartilage defects. At the end of a successful cell transplant, cartilage defects should be replaced by hyaline cartilage that is indistinguishable from its native counterpart. Unfortunately, the transplanted cells often die in the hostile environment of an arthritic joint, which leads to formation of fibrocartilage and/or scars rather than hyaline cartilage. The inability to diagnose cell transplant failure in a timely manner represents a major barrier for the development of effective joint regeneration procedures. An imaging biomarker for stem cell engraftment or failure could improve our ability to identify the best-suited cell type and cell transplant procedure, and in turn could improve the success of novel cell therapies for arthritis treatment. Thus, this project aims to develop an immediately clinically applicable imaging test to differentiate between viable and apoptotic matrix- associated stem cell transplants (MASI) in arthritic joints. We have developed clinically translatable approaches for labeling stem cells using ferumoxytol, an FDA-approved iron oxide nanoparticle compound. We have also patented an imaging technique for non-invasive monitoring of ferumoxytol-labeled cells in arthritic joints. Ferumoxytol-labeled apoptotic MASI demonstrated distinct signal characteristics on magnetic resonance (MR) images compared to viable MASI in small animal models. The current proposal aims to translate these approaches to the clinic. The first aim is to prove that ferumoxytol labeling provides critical information about engraftment or failure of MASI in an immune-competent large animal model. The second aim is to evaluate the biological impact of ferumoxytol labeling on cartilage repair outcomes of human MASI in a severe combined immunodeficiency porcine model. The third aim is to generate proof of concept of diagnosing successful or failed cell transplants in a Phase I clinical trial. The results can be immediately useful to track cell transplants in arthritic joints, understand the impact of iron labeling on stem cell engraftment outcomes, and predict the success of cell transplants in patients with MR imaging. These results can then inform the design of clinical trials, and ultimately improve the success rate of MASI-mediated joint repair. Tracking MASI is important because it improves our understanding of the role of therapeutic cells in tissue regeneration processes. This understanding can be leveraged to optimize cell transplant procedures. The cell tracking technique proposed in this project will provide critical information about MASI engraftment outcomes, accelerate the development of successful cell therapies for joint regeneration, and ultimately help alleviate long- term disabilities and related costs to our society.